Interpretive Summary: Polymorphism among species of medium stomach worms complicates the process of identification, diagnosis, and definition of faunal diversity among these pathogenic nematodes in North American ruminants. Although most polymorphic species have been documented in detail, and occur with some regularity in specific ruminant hosts, until recently Ostertagia kasakhstanica the postulated minor morphotype of O. bisonis had eluded detection. We present the first descriptions of the diagnostic characters useful in identification and differentiation of O. kasakhstanica from other medium stomach worms in wild and domestic ruminants from western North America. Basic information is provided to accurately identify O. kasakhstanica that will be useful to scientists working on the pathogens and parasites of cattle, deer, and other wild ruminants that share common pastures. These studies also serve to reinforce the concept that comparative morphology remains one of the most important tools for identification of pathogenic nematodes in ruminants.

Technical Abstract:
Ostertagia cf. kasakhstanica, the putative minor morphotype of the polymorphic ostertagiine O. bisonis, is reported in Bison bison from South Dakota. We present the first descriptions of the synlophe and details of other diagnostic characters useful in identification and differentiation of this minor morphotype from other ostertagiines in wild and domestic ruminants from western North America. Laterally the cervical synlophe is consistent with Type Ib pattern as defined by Lichtenfels et al. (1988) and is within the range of variation previously described for O. bisonis as predicted by Lichtenfels and Pilitt (1991). Among recognized minor morphotypes of ostertagiines in wild and domestic ruminants from North America, O. kasakhstanica is distinct from O. kolchida, O. dikmansi and O. arctica based on a tapering lateral synlophe. It is further distinguished from O. lyrata by the pattern of the cervical synlophe in the lateral field, structure of the genital cone and spicules and dimensions of the esophageal valve. Although morphologically similar to T. davtiani/T. trifucata and T. boreoarcticus f. minor it can be readily distinguished by the 2-1-2 bursal pattern, the structure of the spicules and genital cone and length of the esophageal valve. Ostertagia bisonis/O. cf. kasakhstanica circulates among pronghorn, mule deer, bison and cattle in west-central North America.